st: running sum

Dear stata people,
Im stuck trying to do a running sum using by. My problem is like:
Suppose we have serveral power distribution lines (composed of
serveral substations) all around the states. Suppose our variables are
LINE SUBST POWER
I want to generate a variable CUM_POW containing the cummulative sum
of POWER by LINE SUBST
For instance,
LINE SUBST POWER CUM_POW
1 1 10 10
1 2 20 30
1 3 15 45
1 4 5 50
how goes the sintaxis for that?
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